The stress disorder, P.T.S.D. for short, is common among combat veterans and others who have endured severe trauma, like 9/11 rescue workers. Previous studies have demonstrated a connection between asthma and psychiatric illnesses, but no one knows whether one disorder increases the risk for the other or whether they share a common risk factor, either environmental or genetic.

Researchers used data on 3,065 male twin pairs who had lived together as children and had active duty in the Vietnam War. They adjusted the findings to eliminate the influence of depression, smoking, age, body mass index, exposure to combat and other variables.

One-fourth of men with the most severe symptoms of the stress disorder were more than twice as likely to suffer from asthma as the quarter with the fewest P.T.S.D. symptoms.

The association cannot be fully explained by familial or genetic factors; identical twins, who have exactly the same genes, were no more likely to suffer from both illnesses than fraternal twins, who share only half their genes.

The researchers, whose paper appears in the Nov. 15 issue of The American Journal of Respiratory and Critical Care Medicine, are still seeking the explanation.